For me its nice good Info for new Users that would start reporting.
So i guess the Old post note was just implemented today or yesterday right , or did i missed something the last days ?
Maybe theymos or hilariousandco or mprep can say something about it.
This warning only appears if you report a post that is older than x months or days (I don't know the exact value), so you probably haven't seen it until now because you haven't reported posts that fall into that category. If you try to report the first post in this topic, you will see that warning - if you try to report one of the recent posts, then of course you will not see anything.
For me personally, such a warning makes sense, because it really doesn't make much sense to dig through years-old posts and report them to mods.